Multiple-Input Multiple-Output (MIMO) is an advanced antenna technique utilized in wireless systems (e.g., cellular communications networks) to improve spectral efficiency and thereby boost overall system capacity. For MIMO, a commonly known notation of (M×N) is used to represent the MIMO configuration in terms the number of transmit antennas (M) and the number of receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (8×4). The MIMO configurations represented by (2×1) and (1×2) are special cases of MIMO, and they correspond to transmit diversity and receive diversity, respectively.
Using multiple antennas at the transmitter and receiver can significantly increase the system capacity. Specifically, transmission of independent symbol streams in the same frequency bandwidth, which is commonly referred to as Spatial Multiplexing (SM), achieves a linear increase in data rates with the increased number of antennas. On the other hand, by using space-time codes at the transmitter, reliability of the detected symbols can be improved by exploiting the so called transmit diversity. Both the SM scheme and the transmit diversity scheme assume no channel knowledge at the transmitter. However, in practical wireless systems such as the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), High Speed Downlink Packet Access (HSDPA), and WiMAX wireless systems, channel knowledge can be made available at the transmitter via feedback from the receiver to the transmitter. The transmitter can utilize this channel information to improve the system performance with the aid of precoding. In addition to beam forming gain, the use of precoding avoids the problem of an ill-conditioned channel matrix.
In practice, complete Channel State Information (CSI) may be available for a wireless system using a Time Division Duplexing (TDD) scheme by exploiting channel reciprocity. However, for a wireless system using a Frequency Division Duplexing (FDD) scheme, complete CSI is more difficult to obtain. In a FDD wireless system, some kind of CSI knowledge may be available at the transmitter via feedback from the receiver. These wireless systems are referred to as limited feedback systems. There are many implementations of limited feedback systems such as, e.g., codebook based feedback and quantized channel feedback. 3GPP LTE, HSDPA, and WiMax recommend codebook based feedback for precoding. Examples of CSI are Channel Quality Indicator (CQI), Precoding Indicator (PCI) (which is also referred to as a Precoding Matrix Indicator (PMI)), and a Rank Indicator (RI). One type of CSI or a combination of different types of CSI are used by a network node (e.g., a base station such as, for instance, a Node B in a Universal Terrestrial Radio Access (UTRA) network or an evolved or enhanced Node B (eNB) in LTE) for one or more resource assignment related tasks such as, e.g., scheduling data transmissions to a User Equipment device (UE), rank adaptation of MIMO streams, precoder selection for MIMO streams, etc.
In codebook based precoding, a predefined codebook is defined both at the transmitter and at the receiver. The entries of the codebook, which are commonly referred to as precoding matrices, can be constructed using different methods, e.g., Grassmannian, Lloyd's algorithm, Discrete Fourier Transform (DFT) matrix, etc. Each precoder matrix is often chosen to match the characteristics of the N×M MIMO channel matrix H for a particular number of transmit antennas (M) and receive antennas (N), resulting in so-called channel dependent precoding, where N≥1 and M≥1. This channel dependent precoding is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
Issues with codebook based precoding in a closed-loop MIMO wireless system arises from the fact that the performance of the system generally improves with the cardinality (i.e., size) of the codebook. Specifically, at the receiver, the receiver must evaluate all possible precoding matrices for all possible ranks for a given MIMO configuration (M×N) and report a RI and a PCI for the best rank and precoding matrix to the transmitter every Transmit Time Interval (TTI) or every few TTIs. Evaluating all possible precoding matrices for all possible ranks is a computationally intensive process. For example, in four branch MIMO in LTE, the UE must search 64 precoding matrices (also referred to as precoding entities) for finding the best rank and precoding matrix. This search of the 64 precoding matrices increases power consumption, drains UE battery life, and consumes more memory and processing resources at the UE. Furthermore the network node serving the UE may not always use a full set of CSI (e.g., a full set of ranks and precoding matrices). In this case, if the UE reports CSI (e.g., a RI and a PCI) out of the full set of CSI (e.g., all possible ranks and precoding matrices), then the network node may need to spend more resources or perform additional processing to identity an appropriate CSI for scheduling the UE.
Systems and methods that address these issues are described in commonly owned and assigned Patent Cooperation Treaty (PCT) International Publication No. WO 2014/027949 A2, entitled IMPLEMENTING CODEBOOK SUBSET RESTRICTIONS IN WIRELESS COMMUNICATION SYSTEMS, which was published on Feb. 20, 2014. In particular, WO 2014/027949 A2 discloses, among other things, embodiments in which a transmitting node provides feedback restriction information to a receiving node, where the feedback restriction information may specify restrictions on a feedback report to be sent from the receiving node. In one particular embodiment, the feedback restriction information is provided by transmitting an unused bit pattern in the five bits corresponding to the modulation information in Part 1 of a High Speed Shared Control Channel (HS-SCCH) in a HSDPA system to indicate that there is a restriction for feedback purposes. The feedback restriction(s) are then transmitted using a bit map in at least some of the remaining bits of the HS-SCCH. In other words, transmission of the unused bit pattern is an indication that at least some of the remaining bits of the HS-SCCH are being repurposed for transmitting feedback restriction(s).
However, there still remains a need for additional schemes for addressing the issues described above for closed-loop MIMO systems utilizing codebook based precoding.